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copied!<p>This code does more or less what you ask for - it is a simple proof of concept:</p> <ul> <li>a proper implementation needs to also deal with the <code>declaredAnnotations</code></li> <li>if the implementation of annotations in Class.java changes, the code will break (i.e. it can break at any time in the future)</li> <li>I have no idea if there are side effects...</li> </ul> <p>Output:</p> <blockquote> <p>oldAnnotation = some value<br> modifiedAnnotation = another value</p> </blockquote> <pre><code>public static void main(String[] args) throws Exception { final Something oldAnnotation = (Something) Foobar.class.getAnnotations()[0]; System.out.println("oldAnnotation = " + oldAnnotation.someProperty()); Annotation newAnnotation = new Something() { @Override public String someProperty() { return "another value"; } @Override public Class<? extends Annotation> annotationType() { return oldAnnotation.annotationType(); } }; Field field = Class.class.getDeclaredField("annotations"); field.setAccessible(true); Map<Class<? extends Annotation>, Annotation> annotations = (Map<Class<? extends Annotation>, Annotation>) field.get(Foobar.class); annotations.put(Something.class, newAnnotation); Something modifiedAnnotation = (Something) Foobar.class.getAnnotations()[0]; System.out.println("modifiedAnnotation = " + modifiedAnnotation.someProperty()); } @Something(someProperty = "some value") public static class Foobar { } @Retention(RetentionPolicy.RUNTIME) @interface Something { String someProperty(); } </code></pre>

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